[LLVMdev] Strange pointer aliasing behaviour

John McCall rjmccall at apple.com
Thu Jun 17 10:58:42 PDT 2010

On Jun 17, 2010, at 10:42 AM, Andrew Haley wrote:

> On 06/17/2010 06:19 PM, Jeffrey Yasskin wrote:
>> On Thu, Jun 17, 2010 at 12:22 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Wed, Jun 16, 2010 at 11:14 PM, Pierre C <lists at peufeu.com> wrote:
>>>>> There are essentially two ways to "solve" this issue: one is
>>>>> type-based alias analysis, i.e. assuming "double" and "int" don't
>>>>> alias; LLVM doesn't implement this at the moment.  The other is to
>>>>> attempt to analyze the loop and prove that %indvar.i is never
>>>>> negative; LLVM doesn't implement anything like this at the moment
>>>>> either.
>>>>> -Eli
>>>> Actually I think it's much simpler than that...
>>>> http://llvm.org/releases/1.3/docs/AliasAnalysis.html#basic-aa
>>>> it says says "Different fields of a structure do not alias."
>>>> This is the case here : we have two different fields of a struct however it
>>>> mistakenly thinks they alias.
>>> Consider a case like the following:
>>> struct X { int a; int b[10]; };
>>> int f(struct X* a) { a->b[-1] = 1; return a->a; }
>>> This is technically illegal code, but various programs depend on
>>> constructs like this working.
>> I don't know if it's illegal, but this is how libstdc++'s string
>> implementation finds its header data. std::string stores a pointer
>> directly to the character data (making subscripting slightly faster),
>> and then subtracts the size of the header when it needs to do any
>> bookkeeping.
> Character types are special: they can alias everything.  if this weren't
> the case you couldn't write malloc().
>> Do you have a reference to the standard that makes it undefined?
> Several places, but of C99 is the most important: it lists all
> the legal pointer conversions.

The better reference is 6.5.6p7-8, about pointer addition:
  For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.
  If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

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